Over the past few decades, there has been a growing interest in informality as a major driver of urbanization and city life around the world. Policy makers, planners, architects, development workers, artists, and activists have focused on urban informality across multiple domains: from housing and settlement, to work, infrastructure, mobility, finance, and other modes of human experience. In most cases, urban informality emerges as a 'problem to be solved' through a variety of interventions. This course takes a close and careful look at urban informality as it unfolds across the world, examining it along several dimensions, including the conceptual, the historical, the material, and the affective. We consider modes of informality across a broad swath of the Global North and Global South, with due attention to informal practices among elites and the middle-class as well as working-class and poor people. Finally, we review multiple efforts over the last half-century to define and intervene into urban informal systems, whether through ideologically-driven programs or well-intentioned technocratic 'improvement' schemes. In the end, the purpose of this course is not to figure out how we can solve "the problem" of urban informality, but rather to figure out how we are conceiving "the problem" in the first place.